(1) Field of the Invention
The present invention relates generally to a time division multiple access system (hereinafter referred to as a TDMA system) and, more particularly, to a monitoring circuit for a descrambling device in a receiving terminal for the TDMA system.
(2) Description of the Prior Art
In general, in a transmitting terminal of a TDMA system, data to be transmitted is scrambled by using a pseudo noise sequence so that the energy of the transmitted data can be prevented from being concentrated in a specific region of the frequency spectrum. In addition, a clock signal can be effectively restored in the receiving terminal. That is, data to be transmitted is exclusive-ORed with the pseudo noise sequence and the resultant signal modulates an IF carrier.
In the receiving terminal, it is necessary to remove the pseudo noise sequence which is used for scrambling, from the received data. Therefore, after the received data is demodulated, the data is descrambled by using the same pseudo noise sequence used in the transmitting terminal, so that the original data is restored.
The transmitting terminal has a pseudo noise generator for generating a pseudo noise sequence for scrambling, while the receiving terminal has a pseudo noise generator for generating a pseudo noise sequence for descrambling. For normal operation of the TDMA system, it is necessary to monitor the descrambling device. It is difficult to monitor the sequence (periodicity) of the pseudo noise sequence generated by the pseudo noise sequence generator of the descrambling device, because the generator is initialized every time burst signals are received.
A conventional monitoring circuit for a descrambling device comprises a retriggerable monostable multivibrator for detecting an output signal of the pseudo noise sequence generator. As a result, whether or not the pseudo noise sequence generator is operated can be determined. However, in the conventional monitoring circuit, whether or not a pseudo noise sequence generated by the pseudo noise sequence generator is correct cannot be determined.